The adaptation of man and other animals to a planet with a 24 hour period of rotation has resulted in the evolution of a circadian timekeeping system (CTS) that organizes the physiology and behavior of organisms. The CTS is essential for daily life; when it malfunctions, other homeostatic and regulatory systems are compromised. Such effects are apparent in the humans suffering from jet-lag or some sleep-wake disorders. It is important to understand the neural organization of the circadian system. This research program will examine various components of the circadian system and elucidate their function. Animals implanted with biotelemetry units to allow for the collection of body temperature, activity and heart rate data will receive small lesions in a subcomponent of the circadian system. The resulting deficits in circadian function will be examined in a controlled lighting and temperature environment. In addition to the physiological data mentioned above, drinking and feeding will also be monitored. Using techniques such as these, we have shown that a neural pacemaker controlling activity-related rhythms is located in the suprachiasmatic nucleus of the hypothalamus. In a time-cue free environment, animals normally live a day that is structured by their internal circadian pacemaker, an animal with lesion of this nucleus lives an arrhythmic day. While they are able to maintain homeostatic levels of activity, the activity is no longer organized into the typical diurnal rhythmic pattern, but small bouts of activity occur scattered throughout the day and night. *KEY*Biological rhythms, Temperature regulation, Feeding, Activity, Drinking